As a rule, austenitic materials which have satisfactory resistance to general corrosion in both oxidizing and reducing media and also to local corrosion have increased chromium and molybdenum contents. It is known that molybdenum exerts a stronger influence than chromium on resistance to local corrosion. This is shown by the calculation of the action sum W=% Cr+3.3% Mo, a value which serves as a yardstick for determining the resistance to local corrosion to be expected from the composition of the alloy. Frequently the alloying element nitrogen is also included with a factor of 30 in the calculation of the action sum, since a positive influence on resistance to local corrosion is also ascribed to nitrogen. However, higher contents of chromium and molybdenum have an adverse effect on the structural stability of the materials and therefore exert a disadvantageous effect on processing behaviour (hot shaping, welding, etc.). One possible way of improving structural stability is to add nitrogen, but this step is limited by the limited solubility of nitrogen in austenitic materials. Moreover, chromium nitrates may become precipitated and have an adverse effect on resistance to corrosion. Maximum conditions of chromium and molybdenum can be adjusted in the materials only if the nickel content is raised in parallel. Due to the lower carbon solubility in materials based on nickel in comparison with steels, however, the carbon activity increases comparatively more strongly in materials based on nickel. To achieve satisfactory resistance to corrosion, more particularly to reduce liability to intercrystalline corrosion, the prior art requires the known nickel-chromium-molybdenum alloy NiMo16CrTi (Material No. 2.4610 in the Iron and Steel List of the Verein Deutscher Eisenhuttenleute; Publishers Stahleisen mbH, 7th Impression, 1981, corresponding to U.S. Material UNS NO6455) must be stabilized with titanium. An addition of vanadium is also required, for example, as a stabilizing element for the known nickel-based materials NiMo16Cr15 (Material No. 2.4819, corresponding to UNS N10276) and also NiCr21Mo14W (Material No. 2.4602, corresponding to UNS NO6022). The Material NiCr22Mo9Nb (Material No. 2.4856, corresponding to UNS NO6625) is stabilized by an addition of niobium. The amount of added contents of said stabilizing elements normally amounts to 10 to 20 times that of the carbon content, but in the case of the material NiCr22Mo9Nb amounts to 50 to 100 times that content. Stabilization (bonding of the carbon) guarantees the improved resistance to corrosion of welded components without any additional heat treatment.
0.25-0.5% titanium is normally added to the material NiMo16CrTi. According to investigations by R. W. Kirchner and F. G. Hodge, published in "Werkstoffe und Korrosion" (Materials and Corrosion), Vol. 24, 1973, pages 1042-1049), in addition to carbon, titanium also bonds nitrogen via the formation of nitrides. By this effect, titanium is intended to reduce the tendency to sensitization of the material, thus facilitating further processing, for example, welding. However, it is a disadvantage that titanium nitrides produced are present scattered in the structure of the material and more particularly with fairly large dimensions may be locally more strongly concentrated in the form of cloud-shaped accumulations. This then results in corresponding unevennesses of the material which under fairly heavy stressing by corrosion and erosion may take the form of locally uneven detrition. As a result the material loses that smooth-walled surface which is required in the course of many processes and is absolutely necessary to avoid caking e.g., the depositing of gypsum in absorbers for flue gas desulphurization.